1. Field of the Invention
The present invention relates to a variable damping-force damper and more specifically to a variable damping-force damper for use in damping vibrations of a vehicle or the like for example as well as to a manufacturing method of the same.
2. Description of Related Art
There is known a variable damping-force damper using a MRF (Magneto-Rheological Fluid) as a working fluid in which spherical particles having an average size of around several μm and ferromagnetism (referred to as “magnetic particle” hereinafter) are dispersed as a dispersed material in oil such as mineral oil, i.e., a dispersant medium (this variable damping-force damper will be referred to as a “MRF damper” hereinafter).
Although the MRF is a liquid similar to a general hydraulic working oil and behaves as a Newtonian fluid when no magnetic filed is applied to the MRF, the magnetic particles dispersed homogeneously within the MRF concatenate along a direction of a magnetic filed and form chain-like clusters when the magnetic field is applied from the outside. Because the clusters resist against deformation (flow), an apparent viscosity of the MRF sharply increases and the MRF behaves like a plastic fluid having yield stress when it flows. Such change of viscosity caused by the magnetic field of the MRF is reversible. It is also possible to control the degree of the viscosity of the MRF by controlling intensity of the magnetic field. This change in state of the MRF occurs very quickly and its response to the change of the magnetic field is in order of several milliseconds.
Generally, the MRF damper having the MRF is constructed as follows. That is, a piston partitions an inside of a cylinder tube filled with the MRF into first and second oil chambers (first and second chambers). The piston is connected with a piston rod that projects out of one end of the cylinder tube. A rod guide that is disposed so as to close one end of the cylinder tube slidably supports the piston rod.
The piston is provided with a communication hole that circulates the MRF between the first and second oil chambers. The piston also contains an electromagnetic coil for applying a magnetic field to the MRF within the communication hole. The viscosity of the MRF may be changed and a variable damping force may be obtained by controlling magnitude of the magnetic field applied to the MRF within the communication hole by power fed to the electromagnetic coil.
The piston slides relative to the cylinder tube during when the MRF damper operates, so that the magnetic particles within the MRF collide against an inner peripheral surface of the cylinder tube and an outer peripheral surface of the piston, causing a rubbing action. Because iron powder and other is used as the magnetic particles, the rubbing action largely affects the cylinder tube and the piston if they are made of materials whose hardness is smaller than iron. That is, sliding surfaces of the cylinder tube and the piston are largely abraded and it becomes difficult to generate the damping force.
Then, U.S. Pat. No. 6,464,051 has proposed a configuration of forming a Ni plating film on an inner peripheral surface of the cylinder tube into which the MRF is charged by implementing an electroless nickel (Ni) plating at first and then forming a Cr plating film by plating chrome (Cr) on the surface of the Ni plating film. While Vickers hardness of the Ni plating film formed by means of the electroless Ni plating is about 550 to 700 VHN (Vickers Hardness Number), Vickers hardness of the Cr plating film formed by means of Cr plating is about 900 to 1000 VHN. Thus, U.S. Pat. No. 6,464,051 suppresses the abrasion of the cylinder tube otherwise caused by the magnetic particles by providing the Cr plating film whose hardness is large on the sliding surface of the cylinder tube.
When the MRF damper is driven, not only the piston slides relative to the cylinder tube but also its piston rod slides relative to the rod guide. Therefore, a technology for keeping sliding resistance (friction) between the piston rod and the rod guide low and for suppressing abrasion of the both sliding surfaces of the piston rod and the rod guide is required.
Then, JPA2000-514161 has disclosed a configuration in which a metal bush that functions as a sealing member and a lubricant member is disposed between the piston rod and the rod guide. More specifically, the metal bush having a structure in which a porous layer made of bronze is provided on an inner peripheral surface of a base material made of non-magnetic metal and in which fluorine resin is immersed in the porous layer is used to slide the fluorine resin immersed layer relative to the piston rod. Note that it is necessary to construct the MRF damper so that no magnetic field acts on the MRF in parts other than the communication hole in order to obtain the variable damping-force by changing the viscosity of the MRF within the communication hole provided in the piston. Therefore, the non-magnetic metal is used as the base material of the metal bush.
Still more, surface roughness of the piston rod is important in the MRF damper from aspects of damping performance and durability of the damper. That is, while the sealing member is provided on the inner peripheral surface of the cylinder tube in order to prevent the magnetic particles within the MRF from flowing out when the piston rod slides in the MRF damper, there is a possibility that the magnetic particles enter irregular portions of the surface of the piston rod and flow out passing through the sealing member when the surface roughness of the piston rod is coarse (i.e., the surface roughness is large).
Then, as a technology of solving such problem, U.S. Pat. No. 6,516,926 has disclosed a technology of providing a Cr plating film on an outer peripheral surface of the piston rod and of polishing and smoothing the surface as a perfectly circular surface by using a tape polishing method or the like.
The cylinder tube contacts with the piston in the MRF damper in a manner of metal-to-metal contact in general and a clearance between the cylinder tube and the piston is required to be very accurate. However, because the Cr plating is electroplating (electrolytic plating), it is difficult to uniformize a thickness of the Cr plating film by controlling Cr plating conditions. The thickness tends to vary even more in forming the Cr plating film on the inner peripheral surface of the cylinder tube. Therefore, the Cr plating film is formed into a thickness fully larger than a thickness that is originally required for the Cr plating and then the Cr plating film is ground and polished to a desirable thickness by way of honing or the like to meet with the required precision of the abovementioned clearance.
When a cylinder tube having a Cr plating film 40 μm thick on its inner peripheral surface is to be manufactured for example, the Cr plating film is formed by Cr plating so that its maximum thickness becomes about 100 μm on the inner peripheral surface of the cylinder tube and then the thickness is adjusted to 40 μm by grinding and polishing such as honing. However, such manufacturing method has problems that productivity drops because it requires a long treatment time for the Cr plating and a certain processing time for grinding and polishing, e.g., honing, the film and that it is costly due to the costs required for the Cr plating and for grinding and polishing, e.g., honing, the film.
In a case of forming a Cr plating film on the piston rod, it is also required to perform a rounding process while reducing surface roughness by polishing the Cr plating film by using a polishing tape or the like after forming the Cr plating film that is fully thicker than a desired thickness by Cr plating in the same manner with the case of the cylinder tube. Accordingly, such manufacturing method of the piston rod has problems that it is costly due to polishing and that productivity drops because it requires a long polishing time.
Still more, a circumstance in which the MRF damper is driven while being biased by a side force for example occurs frequently in a vehicle or the like using the MRF damper. Under such circumstance, there is a problem in terms of durability that abrasion is accelerated because the fluorine resin impregnated layer of the metal bush is soft. Still more, because the magnetic particles contained in the MRF are very small, the magnetic particles infiltrate into the gap between the piston rod and the rod guide when the fluorine resin impregnated layer of the metal bush abrades away and accelerate the abrasion of the fluorine resin impregnated layer further.
Then, as a method for solving such problems, it is conceivable to use a material obtained by impregnating fluorine resin such as tetrafluoroethylene to hard alumite (referred to as “fluorine resin contained hard alumite” hereinafter) on the sliding surface of the rod guide. The fluorine resin contained hard alumite excels in abrasion resistance because it is harder than the fluorine resin impregnated layer of the metal bush and has characteristics that its sliding resistance is small as compared to the hard alumite because it contains the fluorine resin.
However, the inventors found that the fluorine resin contained hard alumite has a problem that it indicates a relatively large sliding resistance value in a state being pressed by a large force. Therefore, the MRF damper structured by using the rod guide having the fluorine resin contained hard alumite on its sliding surface that slides relative to the piston rod has a possibility of dropping accuracy in outputting a target damping force if the MRF damper is biased by the side force or the like when a driving signal (specifically, this indicates magnitude of an electric current flown to the electromagnetic coil and is referred to as an “input signal” hereinafter) is inputted to the MRF damper to obtain a desirable damping force because the sliding resistance between the piston rod and the rod guide increases. Still more, the increase of the sliding resistance between the piston rod and the rod guide tends to advance the abrasion of the sliding surfaces of the piston rod and the rod guide. It further causes various problems that drop the durability of the damper by causing a rickety piston rod supporting state, a leak of the MRF and the like.
The present invention has been made in view of the problems described above and seeks to provide a variable damping-force damper that is capable of suppressing the drop of operational accuracy in the state being biased by the side force or the like and has excellent durability as well as to provide a manufacturing method of the same with high productivity and at low cost.